Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within an outer jacket or envelope. The arc tube, itself hermetically sealed, has tungsten electrodes mounted therein and contains a fill material including mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud which comprises a generally cylindrical light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends. In other cases the shroud is open on one end and has a domed configuration on the other end. Shrouds for metal halide arc discharge lamps are disclosed in U.S. Pat. No. 4,499,396 issued Feb. 12, 1985 to Fohl et al.; U.S. Pat. No. 4,620,125 issued Oct. 28, 1986 to Keeffe et al; U.S. Pat. No. 4,625,141 issued Nov. 25, 1986 to Keeffe et al; U.S. Pat. No. 4,580,989 issued Apr. 8, 1986 to Fohl et al.; U.S. Pat. No. 4,709,184 issued Nov. 24, 1987 to Keeffe et al.; U.S. Pat. No. 4,721,876 issued Jan. 26, 1988 to White et al.; U.S. Pat. No. 4,791,334 issued Dec. 13, 1988 to Keeffe et al.; U.S. Pat. No. 4,888,517 issued Dec. 19, 1989 to Keeffe et al.; and U.S. Pat. No. 5,023,505 issued Jun. 11, 1991 to Ratliff et al. See also U.S. Pat. No. 4,281,274 issued Jul. 28, 1981 to Beehard et al.
The shroud has several beneficial effects on lamp operation. In lamps with a gas-filled outer envelope, the shroud reduces convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to equalize the temperature of the arc tube. In addition, the shroud effectively reduces sodium losses from the arc tube and improves the maintenance of phosphor efficiency in metal halide lamps having a phosphor coating on the inside surface of the outer envelope. Finally, the shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters.
All of the known prior art metal halide lamps which utilize a shroud are single-ended with respect to mounting and application of electrical energy to the arc tube. The shroud is held in position within the lamp envelope by attaching it to a metal frame which extends between the ends of the lamp envelope. Metal clips or straps attached to the ends of the shroud are welded to the frame.
Double-ended metal halide lamps have been developed for low wattage and other special applications. The arc tube is mounted within a light-transmissive outer jacket and the ends of the outer jacket are press-sealed, with the arc tube electrical leads extending through the press seals. The lamp is mechanically supported at both ends, and electrical energy is applied to opposite ends of the lamp. It is desirable to use a light-transmissive shroud in a double-ended metal halide lamp to provide one or more of the advantages described above. However, the shroud mounting techniques used in prior art single-ended lamps may not be suitable for use in double-ended lamps. In double-ended lamps, the space between the outer jacket and the arc tube is very limited. In addition, these lamps operate at high temperatures. There may be insufficient space to mount the shroud using a metal frame and clips or straps. Even if metal mounting elements could be utilized, it is likely that they would be subject to fatigue in the high operating temperatures of double-ended metal halide lamps.
It is a general object of the present invention to provide improved metal halide arc discharge lamps.
It is another object of the present invention to provide double-ended arc discharge lamps having a light-transmissive shroud between the arc tube and the outer jacket.
It is another object of the present invention to provide double-ended arc discharge lamps which can be safely operated without a protective fixture.
It is yet another object of the present invention to provide double-ended metal halide arc discharge lamps which have a high luminous output and a long operating life.
It is yet another object of the present invention to provide double-ended metal halide arc discharge lamps which are small in physical size.
It is a further object of the present invention to provide double-ended metal halide arc discharge lamps which are low in cost and are easily manufactured.